This invention relates to solid-state color imagers, and more particularly to a solid-state color imager provided with color filters.
Recently, as VTRs for industrial or domestic use have come into wide use, the demand for television cameras which are small in size, light in weight and easy of handling has risen. Therefore, notice has been taken of a solid-state television camera employing a semiconductor integrated circuit (in general, IC or LSI). The solid-state television camera is such that the faceplate and the electron beam generating portion of a conventional image pickup tube are substituted by an IC body and form an independent solid-state imager. Since the solid-state television camera uses no electron beam, it is superior to the image pickup tube in the points of high stability, low power dissipation, convenient handling, etc., and it is expected as the television camera of the coming generation.
The solid-state imager has a color filter portion stacked on a semiconductor body portion.
In case of using complementary color filters as the color filters, it is common practice to employ a system in which colors are reproduced with the four sets of cyan, yellow, green and white. In that case, green is produced by the overlapping parts of the filters of cyan and yellow.
In general, the filters are made of an organic material such as gelatine and polyvinyl alcohol. The material is applied onto the semiconductor body, and it is exposed to light through a mask and then developed to be patterned. Thereafter, the dye accepting material for the filter is dyed by immersing it in a predetermined dye liquor. After the filter of the first color has been formed, it is covered with an organic material which is not dyed with a dye liquor, and the same steps are repeated, whereby the filter of the second color is formed.
In this case, the spectral transmittance of the filter greatly varies depending upon the thickness of the layer of the dye accepting material for the filter. In order to realize a good color filter, accordingly, the layer of the filter dye accepting material needs to be formed to a precise thickness. However, in case where the layer of the filter dye accepting material is produced by utilizing a photosensitivity bestowed on the filter dye accepting material as described above, the thickness of the layer of the filter dye accepting material is determined by the quantity of the light in the exposure of the pattern of the filter dye accepting material.
In case where the color filter portion is formed directly on the solid-state imager by such method, reflected light from the surface of a substrate becomes a serious problem in contrast to cases of forming ordinary color filters. This is because the substrate of the solid-state imager reflects a large quantity of light unlike a transparent substrate of glass or the like.
In case of performing the exposure on the solid-state imager, the quantity of light in the exposure is determined by the sum between light entering directly from a light source and light reflected from the substrate surface. Accordingly, when the color filter absorbing irradiation light such as ultraviolet radiation (for example, the color filter of yellow) is formed for the first color and the color filter of cyan is formed for the second color above the color filter of the first color, the filter dye accepting material for cyan receives a smaller quantity of light in its part overlying the yellow filter than in its other part on the supposition that the underlying substance is Si. When the filter dye accepting material, for example, gelatine is used with the quantity of light saturated, a pattern of high precision cannot be formed on account of increased fogging. It is accordingly the most suitable to expose the parent material to a quantity of light immediately before the saturation. At this time, regarding the above example, the thickness of the layer of gelatine undergoes a difference of about 30% between the part overlying the yellow filter and the other part. As a result, a spectral transmittance of the color filter becomes a characteristic curve whose longer wavelength side is further shifted towards longer wavelengths. That is, the spectral transmittance in which green is shifted onto the longer wavelength side is obtained. With the color filter having such spectral transmittance, imaging results in a picture of extremely inferior color reproducibility.
An example in which filters are formed directly of a semiconductor substrate having photosensitive portions is known from "Technical Digest of International Electron Device Meeting", December 1976, p. 400.